1. Field
This application relates generally to wireless communication and more specifically, but not exclusively, to neighbor cell list creation and management for femtocells and other access points.
2. Introduction
A wireless communication network may be deployed over a defined geographical area to provide various types of services (e.g., voice, data, multimedia services, etc.) to users within that geographical area. In a typical implementation, macro access points (e.g., each of which corresponds to one or more macrocells) are distributed throughout a network to provide wireless connectivity for access terminals (e.g., cell phones) that are operating within the geographical area served by the network.
A macro network deployment is carefully planned, designed and implemented to offer good coverage over the geographical area. Even with such careful planning, however, such a deployment may not completely accommodate channel characteristics such as path loss, fading, multipath, shadowing, etc., in indoor and potentially other environments. Consequently, macrocell users may face coverage issues (e.g., call outages and quality degradation) indoors and at other locations, resulting in poor user experience.
To supplement conventional network access points (e.g., macro access points) and provide enhanced performance, low-power access points may be deployed to provide coverage for access terminals over relatively small coverage areas. For example, a low-power access point installed in a user's home or in an enterprise environment (e.g., commercial buildings) may provide voice and high speed data service for access terminals supporting cellular radio communication (e.g., CDMA, WCDMA, UMTS, LTE, etc.).
In various implementations, low-power access points may be referred to as, for example, femtocells, femto access points, home NodeBs, home eNodeBs, access point base stations, picocells, etc. In some implementations, such low-power access points are connected to the Internet and the mobile operator's network via a Digital Subscriber Line (DSL), cable internet access, T1/T3, or some other suitable means of connectivity. In addition, a low-power access point may offer typical base station functionality such as, for example, Base Transceiver Station (BTS) technology, a radio network controller, and gateway support node services.
In practice, femtocells may be deployed with minimal planning. Consequently, it is desirable for femtocells to be capable of self-configuring and self-organizing in terms of choosing available radio resources such as, for example, frequency and physical layer identifiers (e.g., primary scrambling codes (PSCs)), and in terms of identifying neighbor cells. However, creating and managing an accurate neighbor cell list (NCL) at a femtocell in unplanned deployments tends to be a relatively challenging task. In contrast with macro access points, the location of femtocell installations may not be known a priori to the operator. Moreover, a femtocell is not necessarily fixed in location during its operating life. For example, a femtocell may be initially installed near a window in an enterprise and later moved indoors due to interference considerations. As another example, a femtocell installed in an apartment unit may be carried to another apartment in another city. At different locations of femtocell installation, the surrounding radiofrequency (RF) conditions and neighbor access points (e.g., macrocells, picocells and femtocells) will likely be different and, therefore, the NCL at the femtocell should be reconstructed. Furthermore, due to RF mismatch, in which the cells that are seen at the femtocell may be different from the cells seen by an access terminal served by the femtocell at various locations of the access terminal, the NCL at the femtocell may not be sufficiently accurate (e.g., for conditions near the outer boundaries of the coverage area).
It is important that the NCL, at the femtocell, is configured correctly. That is, an NCL (e.g., for intra-frequency, inter-frequency and inter-RAT) should contain the physical layer identifiers of all nearby access points (macrocells, picocells, and femtocells).
Incorrect configuration of an NCL may result in poor performance in idle and active mode mobility. Regarding active mobility, an access terminal that is connected to the femtocell and moving out of the femtocell coverage area will likely experience handover failures and call drops. Regarding idle mobility, an access terminal that is camping on the femtocell and moving out the femtocell coverage may briefly go out-of-service during which it cannot receive any pages from the network.
Furthermore, providing an accurate construction of radiofrequency (RF) interference characteristics in the desired femtocell coverage area may require having a correct NCL at the femtocell. An incorrect NCL may result in incorrect representation of macrocell RF interference characteristics in the desired femtocell coverage area. This is applicable to all methods that rely on access terminal reports to construct a macrocell RF interference profile in the surrounding area. For instance, macrocell RF information is used in femtocell downlink transmit power calibration algorithms, in algorithms that limit uplink interference to macrocells by capping femtocell access terminal transmit power level, and so on.